The present invention relates to a new and improved apparatus for determining the pressure of capillary water in the ground, especially in soil.
In its more particular aspects the present invention relates to a new and improved apparatus for determining the pressure of capillary watering the ground, especially in soil, which contains a probe tube adapted to be partially inserted into the soil and which can be filled with a measuring liquid. The probe tube is closed at that one of its ends, which is intended to be inserted into the soil, with a porous plug which is permeable for the measuring liquid. The probe tube is provided at its other end with a removable closure member which is particularly suitable for filling of the measuring liquid. The apparatus also is provided with a pressure measuring device for measuring the pressure within the probe tube above the measuring liquid.
Under the expression capillary water as mentioned above and in the following disclosure there is to be understood the non-absorptively combined portion of the water which is adherent to the soil. Such water forms the living space for aquatic soil organisms and can be absorbed through the roots of plants growing in the soil. The knowledge of the pressure of the capillary water, therefore, among other things, provides important information about the living conditions of the plants, so that such measurements are conducted for the purpose of observing changes in the capillary water resources within limited regions as well as for investigating the amount and the direction of flow of capillary water within relatively large regions.
Usually, relatively simple pressure gauges are used for measuring the pressure of capillary water. In one practical design the pressure gauge consists of a probe tube intended to be inserted into the ground and having a lower end which is closed by a porous plug formed of ceramic material. At the region of the upper end of the probe tube there is arranged a pressure measuring device. Usually, the pressure measuring device is a simple mercury manometer including a manometer tube, the upper end of which communicates with the upper end of the probe tube and the lower end of which is immersed into a mercury-filled container. During use of such a pressure gauge the probe tube is immersed into the ground until the plug reaches a predetermined depth. Thereafter, the probe tube is filled with water and the feed opening is closed in an air-tight fashion by a closure member. Water will exit through the porous plug in such an amount that the weight of the water column in the probe tube is equal to the weight of the mercury drawn into the manometer tube. The height of the drawn-in mercury column then corresponds to the starting point for the measurement. Afterwards, the ground surrounding the plug will additionally draw water through the porous plug of the probe until the subpressure or negative pressure originating therefrom is in equilibrium with the pressure of the capillary water. Such negative pressure corresponds to a further rise of the mercury in the manometer tube, and thus, can be indicated in a simple way as the height of the mercury column above the preceding defined starting point.
The disadvantages of such a simple pressure gauge are believed to be quite obvious. The accuracy of reading the pressure can not be randomly enhanced to any desired extent due to the meniscus of the mercury column in the manometer tube and possibly also in the mercury container as well as due to the hardly unavoidable parallax error. Therefore, reading errors of at least 1 mm Hg have to be tolerated. Furthermore, for the measurement of capillary water pressure in limited as well as in wide or large regions there are usually used several hundred pressure gauges. For the manometers thereof a relatively large amount of mercury will be required which is unpleasant to handle and harmful to the environment. The two last-mentioned disadvantages are aggravated by the fact that for measuring the capillary water pressure, generally the pressure gauges are inserted into the ground for more than one year and the areas to be investigated are only slightly protected, or, not at all, against trespassers or intruders.